The present invention relates to polyols useful in a process for the production of polyurethane foams with hydrohalocarbon blowing agents.
Processes for the production of rigid polyurethane foams are known. Sucrose-based polyols are of particular interest as the primary isocyanate-reactive reactant because of their relatively low cost and because they are relatively simple to produce. Processes for producing such sucrose-based polyols are disclosed, for example, in U.S. Pat. Nos. 3,085,085; 3,153,002; 3,222,357; and 4,430,490. Each of these patents teaches that the disclosed polyols are useful in the production of polyurethane foams.
At the present time, a major concern of foam producers, particularly rigid foam producers, is the development of rigid foam systems in which the chlorofluorocarbon blowing agent is replaced with a more environmentally acceptable blowing agent. HCFCs (i.e., hydrogen containing chlorofluorocarbon) and blends of HCFCs with other materials are presently considered to be possible alternatives.
U.S. Pat. No. 4,900,365, for example, teaches that a mixture of trichlorofluoromethane, a dichlorofluoroethane selected from a specified group and isopentane is useful as a blowing agent for the preparation of polyurethane foams. Dishart et al's paper entitled "The DuPont Program on Fluorocarbon Alternative Blowing Agents for Polyurethane Foams", Polyurethanes World Congress 1987, pages 59-66 discusses the investigation of various HCFCs as possible blowing agents for rigid polyurethane foams. Neither of these disclosures, however, teaches a process for the production of rigid polyurethane foams having good physical properties from sucrose-based polyols only with an HCFC blowing agent. In fact, Dishart et al teaches that conventional sucrose-based polyols produced foams which became soft, shrank and in some cases collapsed when HCFC-123 was used as the blowing agent. It was only when the sucrose-based polyol was used in combination with a urea-based polyol that a relatively stable foam having good properties obtained.
One of the problems encountered with HCFCs is that they tend to degrade under foam-forming conditions to a greater extent than their chlorofluorocarbon predecessors. The hydrohalocarbons undergo dehydrohalogenation to form halogenated alkenes. They may also undergo reduction reactions in which halogen atoms are replaced with hydrogen.
One solution to the HCFC degradation problem which was suggested by Hammel et al in their paper entitled, "Decomposition of HCFC-123, HCFC-123a, and HCFC-141b in Polyurethane Premix and in Foam", was to wait to add the HCFC to the foam-forming mixture until just before use. This solution is not, however, practical in commercial foam production processes.
Means for stabilizing hydrohalocarbons under foam-forming conditions have therefore been sought by those in the art. U.S. Pat. No. 5,137,929, for example, teaches that inclusion of certain types of stabilizers in a foam forming mixture reduces the amount of decomposition of hydrohalocarbon blowing agent during the foaming process. Among the materials taught to be useful as stabilizers are esters, organic acids, anhydrides, aminoacids, ammonium salts, bromoalkanes, bromoalcohols, bromoaromatic esters, chloroalcohols, nitroalkanes, nitroalcohols, triarylmethyl chlorides, triarylmethyl bromides, 3-sulfolene, zinc dialkyldithiophosphate, haloalkyl phosphate esters, carbon molecular sieves, powdered activated carbon, zeolite molecular sieves, sulfonate esters, and haloalkyl phosphate esters. This patent does not, however, teach or suggest that the polyol employed in the foam forming mixture be modified to stabilize the hydrohalocarbon blowing agent.
In their paper entitled, "Minimization of HCFC-141b Decomposition in Rigid Polyisocyanurate Foams", Bodnar et al take a different approach. Bodnar et al recommend that the catalyst employed in the foam forming reaction be selected so than any compatibilizer present in the polyol will not be able to solvate the cation of the catalyst and thereby render the anion of the catalyst more reactive.
Nowhere in the prior art is it taught or suggested that use of a sucrose-based polyol in which some of the hydroxyl groups were blocked would substantially reduce or eliminate the degradation of hydrohalocarbon blowing agents.